As a data output apparatus of word processors, personal computers, facsimile machines and so forth, printers capable of printing desired information such as texts and images on a sheet-type printing medium, e.g., paper, film and the like, are widely utilized.
Although various printing methods are available for such printers, recently an inkjet printing method has particularly attracted attention because of its capability to perform non-contact printing on a printing medium such as paper, ease of color printing, and low noise. Moreover, for a configuration of such printer, in general a serial printing method is widely adopted because of its low cost and ease of downsizing. According to the serial printing method, a printhead discharging ink in accordance with desired printing data is attached to a carriage and printing is performed by reciprocally scanning the carriage in a direction crossing to the conveyance direction of the printing medium (e.g., paper).
Many of the inkjet printers perform discharge irrespective of printing, which is called preliminary discharge. Note that preliminary discharge is to preliminarily discharge ink for recovery of the discharge state aside from the actual image data printing, and means a discharge operation that does not contribute to image printing. There are two main reasons to perform preliminary discharge.
The first reason is that, when printing is not performed for a long time, a volatile component (solvent) of ink evaporates from the nozzle end of the printhead and ink viscosity increases, causing discharge failure. To prevent such discharge failure and deterioration in printing quality caused by the discharge failure, preliminary discharge is performed.
Secondly, an inkjet printer generally performs suction recovery operation on regular basis to prevent ink solvent from evaporating from the nozzle end and to avoid discharge failure. To perform the suction recovery operation, the nozzle portion of the printhead is covered with a cap member and sucked by a pump so as to generate negative pressure in the cap member. As a result, ink in the inkjet head is eliminated through the nozzle. In a case of a printer which comprises a printhead for discharging plural colors of inks for color printing, if a nozzle that discharges plural colors of inks is covered with one cap for suction, the sucked ink is mixed in the cap, attached to the printhead orifice surface, and sucked back to the nozzle, causing color mixture in the nozzle. Such color mixture of ink can also occur when the discharge surface is cleaned (wiped) by a cleaning blade or the like.
To avoid printing with the mixed color ink, a countermeasure utilizing preliminary discharge is widely adopted. More specifically, the mixed color ink is eliminated by performing discharge irrespective of printing.
The preliminary discharge is performed immediately before the start of printing or during print execution. More specifically, immediately before the start of printing, preliminary discharge is performed to discharge unnecessary ink from the nozzle end portion, and the nozzle is filled with normal ink before starting the printing operation. During printing, a time period during which normal discharge is possible is calculated based on conditions such as printhead temperature, temperature and humidity in the printer, and so on. Preliminary discharge is performed at regular intervals that are equal to or shorter than the calculated time period.
Moreover, there is a known technique to perform preliminary discharge of microscopic dots, which are microscopic (small) enough so as not to affect the appearance, at microscopic density in a printing area (hereinafter referred to as preliminary discharge on a sheet). For instance, according to a known technique, preliminary discharge is performed on a sheet of printing paper immediately before discharging ink for image formation (e.g., Japanese Patent Application Laid-Open No. 06-040042).
To assure prevention of discharge failure and ink color mixture, it is necessary to increase the ink discharge amount of preliminary discharge or increase the frequency of preliminary discharge.
However, if the ink discharge amount of preliminary discharge is increased, the amount of wasted ink increases and the amount of ink that can be used in actual printing decreases, resulting in an increased ink cost. Furthermore, if the frequency of preliminary discharge is increased, the throughput decreases and the printhead life shortens due to an increased number of discharges.
Meanwhile, when a preliminary discharge is performed on a sheet of printing paper, little influence is imposed on the image quality in a case where the dots formed on the paper by preliminary discharge are microscopically small and the number of the dots is small, but unwelcome effects ensues in the image quality in a case where the ink discharge amount and the frequency of preliminary discharge are increased.